Another way of checking the volume of a frustum is that it is a cone minus a cone taken off the apex of the original cone. Since a cone is a self-similar figure, the ratio of the top radius of the frustum to the base radius tells you the ratio of the height of the cone to be removed to the height of the original cone. As the cone is a three-dimensional figure, the volume of the removed cone will have a ratio to the original cone which is the cube of the ratio of the removed height to the original cone's height. It sounds more complicated in words than what you have to do geometrically. (Self-similarity is a wonderful thing...)